"The Mosquito"

I read about this over at Joe.My.God on Friday - interesting stuff.  From Wikipedia:

The Mosquito or Mosquito alarm is an electronic device, used for solving loitering problems, which emits a sound with a high frequency. The sound can typically only be heard by people below 25 years of age, as the ability to hear high frequencies deteriorates in humans with age (a phenomenon known as presbycusis). The device is marketed as a safety and security tool for preventing youths from congregating in specific areas.

Some adolescents have found the once annoying sound can be used as a tool, and turned it into a mobile phone ringtone. This prevents disciplinary actions that may be imposed by teachers hearing their phone ring during class.

See/hear for yourself ...